In automotive blower motor installations, several discrete blower speeds are customarily achieved through the use of various resistor elements connected in series with the blower motor via a multi-position speed control switch. In such installations, the resistor elements may be formed in an array on a support structure disposed in the inlet or discharge air path of the blower to dissipate heat generated by the passage of motor current through the resistor elements. The resistor elements may be discrete (wire-wound, for example) or printed in an array on a substrate in the manner of a printed circuit board. In the U.S. Patent to Yamamoto et al. 5,000,662, for example, the resistors are defined by a serpentine pattern of resistive material deposited on a porcelain enameled metallic substrate.
A fundamental engineering trade-off occurs in the design of a resistor array due to conflicting cost, heat transfer and packaging considerations. For packaging considerations, the resistor array should be as small as possible. This also tends to minimize noise and airflow restriction in the air duct. For heat transfer considerations, the array should be designed to maximize heat rejection. In addition to materials selection, this consideration impacts the package size and shape since heat rejection increases with increasing surface area. Of course, cost considerations tend to dictate against the use of exotic materials and complex manufacturing processes.
The current design philosophy in blower motor resistor arrays is to incorporate a metallic element into the array in order to provide adequate heat rejection while keeping the size of the array reasonably small. In Yamamoto et al., for example, porcelain enamel coating is deposited over a metal substrate. In other designs, a metallic fin is attached to the array. These designs may satisfy heat transfer and packaging considerations, but adversely impact cost considerations.